Delayed action fuse



April 5, 1938. E. F. KIEFER DELAYED ACTION FUSE Filed Sept. 20, 1934INVENTOR EDWIN FK/EFER BY r ATTORNEY Patented Apr. 5, 1938 1 UNITEDSTATES ATENT OFFICE DELAYED ACTION FUSE Application September 20, 1934,Serial No. 744,761

16 Claims.

The invention relates to fuses such as those employed to open electriccircuits when the current therein exceeds a predetermined value, andmore particularly to fuses of the delayed action type which are adaptedto carry a relatively large overload for a short period of time or carrya relatively small overload for a longer period of time;

Fuses of this type have been proposed in which the fusible element wassurrounded by a heater element composed of a coil of relatively highresistance wire in series with the fusible element, but such fuses areusually adapted to carry only a small amount of current, and also, dueto the oxide coating that forms on the fusible element, theircharacteristics change with age and they soon become unreliable.

Fuses of larger size have also been proposed in which two conductiveelements were joined by a fusible element suitably alloyed thereto andthe fusing heat was provided by the passage of current through adjacentgranular carbon. However, due to the variation in resistance of suchcarbon under different degrees of compression it is extremely difficultto commercially manufacture fuses of this type that are uniform inaction. I

Fuses of.thls type have also been proposed in which one end of a fusiblewire is soldered to one end of a graphite resistance element. In thisconstruction, however, the solder and the fuse wire are both fusibleand, due to the high conductivity of graphite, a relatively smallcross-section must be used to provide the necessary resistance. As boththe amount of solder used and the amount of fuse wire melted down duringthe soldering operation varies considerably, the amount of fusible metalin contact with the heater element likewise varies and this causesvariations in the action of the fuses.

Therefore, the chief objects of this invention are to provide a fuse ofthe type described that is lower in cost, easier to vmanufacture, moresimple in construction, better adapted to the variin the various typesof fuse containers, and subject to less change in operatingcharacteristics with age. These, and any other new and novel featureswill be more fully described in the following specification and shown inthe accompanying illustration, in which:

Fig. l is a sectional view of a fuse embodying the invention mounted ina plug type fuse container; and,

, Fig. 2 is a sectional view of a similar fuse mounted in a cartridgetype container.

The invention, as illustrated by the drawing, may be embodied in a fuseconsisting of a suitable refractory conductive element I0 providedsuitably tensioned conductive spring such as It or I8 is embedded. Thisfuse may be mounted in any'suitable or conventional holder or containersuch as a plug type holder 20 or a car tridge type holder 22. Theheating element 10, the fusible material it, and the spring such as 16or ill may be electrically connected in series so that the passage ofmore than a predetermined amount of current through the fuse willproduce sufficient heat in the heating element to fuse or soften thefusible material and release the embedded end of the tensioned springthereby breaking the circuit.

The heating element l0 may be of any suitable refractory conductivematerial such as carbon or the like that melts at a temperature abovethe fusing point of the fusible material I 4, and has sufficientinternal resistance to cause the generation of sufficient heat to meltthe fusible material when a current above the carrying ca-- pacity ofthe fuse is passed therethrough. The size and shape of the heatingelement may vary according to the mounting space available and theoperating characteristics desired, although a fuse suitable forprotecting electric equipment and circuit conductors, such as those usedat present, may be provided with a heating element of cylindricalcontour having flat ends in which the length of the element issubstantially twice as great as the diameter.

The carbon rod, forming this heating element may be provided with asuitable recess such as I! in which the fusible material may beembedded, and this recess may be located in any suitable portion such asone end. The size and relative proportion of this recess may varyaccording to operating requirements, but a suitable heating element maybe provided with a flat bottomed cylindrical recess having a diameterand a depth substantially half the diameter of the rod. In general,however, the larger the recess with respect to the rod the lower thecarrying capacity of the fuse due to the reduced sectional area of thecarbon, and the greater the delay in opening the circuit on relativelysmall overloads due to the longer time required to melt or fuse thelarger mass of fusible alloy.

In order to provide a heating element of suflicient size to hold asuitable mass of fusible alloy, and also of suflicient internalresistance to suitably fuse this mass, the carbon rod may be providedwith a resistance increasing recess such as 24 in the end oppositerecess l2. Although the relative size and proportion of this recess mayvary with the operating characteristics, a suitable recess may becylindrical in contour with a diameter substantially half that of thediameter of the rod and a depth somewhat greater than I lead and tin.

The separating springs iii and it may be of any suitable conductivematerial such as steel or bronze and of any suitable size that willcarry the current for which the fuse is intended. The

outer contour of the material is not important and therefore the shapeof the material may be of any form that facilitates manufacture althoughround is preferred. The size and shape of the formed springs may alsovary according to the size and shape of the available space of theparticular container or housing in which the fuse element is mounted. Ingeneral, a spring such as shown at it is suitable for elements mountedin a plug type container.

This improved fuse element may be mounted in any suitable holder ormounting such as a fuse plug 2E3 which in general may be of conventionaldesign comprising a body of dielectric material having a suitable axialopening 28 closed at one end by a suitable transparent memher or windowit, and at the other by a dielectric support 3b which is secured to thebody by a threaded conductive shell forming one terminal of the fuse.conductive member 3% forming the other terminal of the fuse is suitablymounted in the support iii so that a portion thereof is exposed withinthe opening 26. The fuse element may be mounted within this container byany suitable means. For example, the open end of the heating element iiimay be secured to the inner end of central terminal 34 by suitable meanssuch as a joint of solder, and body 25 may be provided with a lateralopening 36, adjacent the upper end of shell 32, through which spring l6extends outward to this shell with which it is joined by a suitablejoint such as solder. This opening 36 is preferably above the normal orfree position of spring IS, a distance somewhat greater than that whichthe inner end of the spring is embedded in the fusible material M sothat the embedded end of the spring will be separated from the fusiblematerial when it fuses.

The improved fuse element may also be mounted in a suitable orconventional cartridge type holder comprising conductiveend caps 38 and40 on opposite ends of a dielectric cylinder or tube 42 by securing theopen end of the heating element in to the inner surface of one cap, suchas 38, by a suitable joint such as solder, and securing the outer end ofspring I8 to the other cap, such as 40, by suitable means such assolder. be provided with an axial opening through which a relativelylong end of spring l8 may project while this cap is being assembled totube 42, after which the spring may be suitably tensioned, soldered tothe cap, and the protruding end removed.

As an example of the operating characteristics of this improved fuse, acarbon heater element cylindrical in outline, /4 inch in diameter and /2inch long, having a flat bottomed recess 32, inch in diameter and inchdeep, substantially filled with fusible alloy, and a flat bottomedrecess 2 of similar size with a wall inch in thickness between therecesses, will carry 5 /21 For convenience in assembly cap 40 mayamperes of current indefinitely and about 20 amperes for a short periodof time, but a current of 7 amperes will cause the fuse to open thecircuit after it has carried this current for about 6 minutes. Also,these fuse elements are relatively-easy to manufacture at acomparatively low cost, they are uniform and reliable in action, and theoperating characteristics do not change with age.

When desirable the operating characteristics of the fuse may be modifiedsomewhat by surrounding the thermal element with a powdered,non-conductive filling material 44 which in general' increases thedelayed action of the fuse. However, a filling material having either ahigher or a lower heat conducting capacity than that of air may beemployed according to the result desired.

Examples of suitable filling materials are finely divided, fluffy, heatabsorbing materials such as magnesium oxide and calcium hydrate. Thesematerials absorb appreciable quantities of heat and thereby increase thedelay action of the fuse without otherwise affecting its carryingcapacity or other characteristics. Although these materials may besupported around the fuse element by any suitable means, the spacewithin the fuse container around the fuse element is usually filled witha material of this type, and preferred results are obtained when thecontainer exerts no compressive force on the filling material so that itis not compressed or compacted. In this condition the filling materialabsorbs sufficient heat for the purpose intended and at the same timeoffers no detrimental resistance to the action or movement of theseparating spring.

The size, form, and relative location of the component elements of thisimproved fuse may be different from that shown, and various similar andequivalent materials may be employed in place of those defined withoutdeparting from the real invention or the desirable and useful resultsobtained.

I claim:

1. A delayed action fuse comprising a refractory conductive heatingelement provided with a recess therein; a fusible alloy secured directlyto the wall of said recess; and a tensioned conductive spring having oneend thereof embedded in said alloy.

2. A delayed action fuse comprising a suitable mass of fusible alloy; asupport for said alloy comprising a. heating element of conductivematerial having a melting point above that of said fusible alloy and arecess therein in which said fusible alloy is directly embedded; and asuitably tensioned spring of conductive material having one end thereofdirectly embedded in said mass.

3. A delayed action fuse comprising an elongated heating element havingan alloy recess in one end thereof and a resistance increasing recess inthe other end thereof; a suitable mass of fusible alloy embedded in saidalloy recess; and a tensioned spring of conductive material having oneend thereof embedded in said mass.

4. A delayed action fuse comprising an elongated earbon heating;-element having an alloy recess in one end thereof and a resistanceincreasing recess in the other end thereof with a separating walltherebetween; a suitable mass of fusible alloy embedded in smd alloyrecess; and a suitable conductive spring having one end thereof securedunder tension to said. mass,

5. delayed action fuse comprising a cyiindrical carbon heating elementhaving a length substantially twice that of its diameter, and providedwith a cylindrical recess in one end having a diameter .and depthsubstantially half the di-' ameter ofsaid element; a fusible alloydirectly secured to the wall of said recess; a tensioned spring havingoneend thereof directly secured to said alloy; and a suitable containerfor said element and spring.

6. A delayed action fuse comprising a' cylindrical carbon heatingelement having a length substantially twice the diameter thereof, acylindrical recess in one end thereof having a diameter and depthsubstantially half the diameter of said element, and a recess in the.other end thereof having a diameter substantially half and a depthsomewhat greater than the diameter of said element; a fusible alloysubstantially filling the smaller of said recesses; a tensioned springhaving one end thereof operatively secured to said alloy; and a suitablecontainer for said heating element and spring.

7. A delayed action fuse comprising an elongated heating elementprovided with a recess therein and also with means for increasing theresistance thereof; a mass of fusible metal embedded in said heatingelement in direct contact with the wall of said recess; a container inwhich said heating element is rigidly secured in electrical contact withone terminal thereof; and a tensioned spring having one end thereofdirectly secured to said fusible metal and the other end thereofsuitably secured to the other terminal of said container.

8. A delayed action fuse comprising an elongated cylindrical carbonheating element provided with a recess in each end thereof with aseparating wall therebetween; a suitable mass of fusible alloy embeddedin one of said recesses in direct contact with said carbon; a containerin which said heating element is rigidly secured with the open endthereof in electrical contact with one terminal of said container; and asuitably tensioned metal spring having one end thereof embedded in saidfusible alloy and the other end thereof operably secured to the otherterminal of said container.

9. A delayed action fuse comprising an elongated carbon heating elementhaving a recess therein; a, suitable mass of fusible alloy directlyembedded in said recess; a hollow container provided with a lateralopening and two circuit terminals in which said element is rigidlysecured in electrical contact with one of said terminals; and aresilient conductive member having one end thereof in said opening inelectrical contact J with the other of said terminals and the other endthereof directly embedded under tension in said fusible alloy.

10. A delayed action fuse comprising an elongated carbon heating elementhaving a recess therein; a suitable mass of fusible alloy directlyembedded in said recess; a hollow container for said element having alateral opening therein a suitable distance -above said fusible alloy;and a resilient conductive member having one end thereof secured in saidlateral opening and the other end thereof directly embedded in saidfusible alloy in a position a greater distance from the free position ofsaid spring than the-distance said end is embedded.

if. A delayed action fuse comprising an eion= gated carbon heating,element having a recess therein; a suitable mass of fusible alloydirectly embedded in said recess; a hollow container for said elementhaving a hollow cylindrical body of dielectric material with aconductive cap .on each end thereof in which said element is rigidlysecured in electrical contact with one of said caps; and a coiled springwithin said container having one end thereof secured to the other ofsaid caps and the other end thereof directly embedded in said alloy in aposition a greater distance from the free position of said spring thanthe distance said end is embedded.

12. A delayed action fuse comprising a suitable container having anopening therein; a heating element mounted in said opening; a recess insaid heating element; a delayed action fusible element suitably mountedwithin said recess; and means for increasing the delay action of saidelement comprising a filling of calcium hydrate within said openingaround each of said elements.

13. A delayed action fuse comprising a suitable container having anopening therein; a fusible element suitably mounted within said openingand comprising an electrically conductive heating element having arecess therein, a mass of fusible material within said recess, and atensioned conductive spring having a portion thereof embedded in saidfusible material; and a nonconductive filling of magnesium oxide withinsaid opening around said fusible element to increase the delay action ofsaid fuse.

14. A delayed action fuse comprising a suitable container having anopening therein; a fusible element suitably mounted within said opening1 ,ded in said fusible material; and a non-conductive filling of finelydivided fluffy material within said opening around said fusible elementto increase the delay action of said fuse.

15. A delayed action fuse comprising a suitable container'having anopening therein; an electrically conductive heating element suitablymounted within said opening and provided with a recess therein; a massof fusible material embedded in said recess; a suitably tensionedconductive spring operably mounted within said opening with one endthereof embedded in said fusible material; and means for increasing thedelaying action of said heating element comprising a suitable filling ofnon-conductive powdered material loosely filling the residual space insaid opening.

16. A delayed action fuse comprising a container; a substantially hollowheating element rigidlysecured within said container and provided with arecess, said heating element being provided with a relatively largeouter surface exposed within said container; a fusible material securedin said recess; a resilient conductor under tension having one endthereof embedded in said fusible material; and means for increasing thedelay action of said fuse comprising a filling of heat conductingmaterial in contact with the outer surface of said heating element fordissipating part of the heat generated.

' therein.

